How does a Mercury Vapour Lamp work?
A mercury vapour lamp has at its centre a small quartz arc tube filled with argon gas under high pressure. This contains a tiny drop of mercury and tungsten electrodes. When electricity passes through the arc tube the mercury is vapourised and emits radiation - heat, light and a great amount of ultraviolet light. The principle is similar to that used in a fluorescent tube, although the radiation produced is very much more intense.

The quartz arc tube permits the ultraviolet light (and the visible light) to pass through it into the main body of the lamp, which is shaped and coated in such a way that this light is then reflected towards the front of the lamp and out of the cover glass, which must of course be of a type permitting UVB to pass through it.

Self-Ballasted and Externally-Ballasted Lamps
Like a fluorescent tube, a mercury vapour arc tube needs a ballast to accurately regulate its electrical input. “Externally-ballasted” lamps (EB lamps) have an electronic ballast box, like a fluorescent tube controller. “Self-ballasted” (SB) lamps use, instead, a tungsten filament inside the lamp, similar to that in normal incandescent lights. This is mounted above or surrounding the arc tube. These “self-ballasted” lamps produce additional visible light and heat from the glowing filament when in use.

This has advantages. A self-ballasted lamp can provide heat, light and UV light all at once. If the beam is diffuse and wide, a large warm well-lit zone could, in theory, be created. If the beam is narrower and more focused, it could, in theory, penetrate further into a large vivarium and create a bright, warm basking spot at some distance from the lamp.

It also has disadvantages. Incandescent filaments are fragile. Whereas fluorescent tubes and EB mercury vapour lamps may last for years, filaments do break and burn out quite easily, particularly if the lamp is subject to vibration or frequent switching. The advertised lifespan of SB lamps varies from “one year of normal daytime use” to “10,000 hour average life” – i.e., over two and a half years at 10 hours a day - but one independent report has suggested that the average lifespan of one of these, in everyday use, may be only about six months.

No thermostat, so watch the heat...
A mercury vapour lamp cannot be dimmed, and does not respond well to frequent switching. In fact all these lamps are designed to cool right down before they will re-start, if the circuit is broken. This means that they cannot be controlled by a thermostat of any sort. All emit a considerable amount of heat and the lowest wattage SB lamp currently manufactured runs at 100 watts.

SB lamps, in particular, are therefore not very suitable for small vivaria, which they could easily overheat. In a large vivarium or enclosure, overheating is less likely to be a problem, and in fact most large scale set-ups would need another form of heating, under thermostatic control, in addition to the lamp -not least because heat of some sort is often needed outside of the hours for which light and UVB are to be supplied.

Types of Lamp available
Mercury vapour lamps are available in two distinct types, the “true flood” lamp, which emits a diffuse beam over a wide area, and the “narrow flood” and “spot” lamps which have a beam which is focused in front of the lamp and reaches much further into the vivarium. The UVB output of these is so different that we will consider each group separately.

What are Mercury Vapour Lamps most suitable for?
Most reptile keepers using mercury vapour lamps reserve them for use in open enclosures or large vivaria (i.e., at least 4 - 5 ft. across) which are well ventilated, to allow hot air produced by the lamp to escape and prevent unwanted heat buildup.

Most brands of mercury vapour lamp produce considerably more UV light than fluorescent tubes and compact lamps. This makes them ideal for species known to bask, in the wild, in full sunshine for at least part of the day. (See our feature, What UV light do reptiles need? )

Some brands produce far-reaching beams of UV light and are particularly useful for creating basking spots some distance from the lamp. Others produce wide beams which can be used to create a large basking area - very suitable for large reptiles which can sit with most, if not all of their body in the "artificial sunlight".

Unsuitable?
Mercury vapour lamps are not suitable for reptiles with low UVB requirements, such as those which rarely bask, or live in shady environments in the wild. They are also unsuitable for most small enclosures; and not just because of the problems with unwanted heat. The intensity of ultraviolet light, like visible light, falls off rapidly with distance. Hence, since mercury vapour lamps are designed to produce high UVB at a distance, they emit higher levels still, at close range.

Manufacturers always state a minimum safe basking distance; for many lamps it is 12" or 18" from the face of the lamp. Closer than this, your reptile may receive dangerously high levels of UV radiation.

It's also adviseable to keep your lamp directly above the basking spot - overhead, like the sun. The shape of a reptile's head and position of the eyes ensures that overhead light will not cause it to suffer from the glare.

A mercury vapour lamp which needs to be placed a minimum of 12" directly above a basking spot, just won't fit inside most small set-ups - there isn't enough headroom. However, if the vivarium has a wire mesh or screen top, the lamp can be hung outside the viv, at a suitable distance above the mesh, and the beam directed down to a basking spot below. Bear in mind, though, that wire meshes and screens will block a percentage of the UV light, as shown in our mesh test results. (Glass or plastic sheets will completely block the rays and thus cannot be used.)

Always check basking temperatures right underneath a mercury vapour lamp.